(a) Field of the Invention
This invention relates to an output transistor protection circuit in a power amplifier, and more particularly, to a protection circuit with a function of limiting a collector power dissipation of an output transistor to a predetermined magnitude so as to protect the output transistor from the destruction thereof.
(b) Description of the Prior Art
A prior art output transistor protection circuit with collector power dissipation limitation limits an input signal magnitude of an output transistor to a predetermined magnitude based on the detection of an output voltage level of an amplifier and a current level of the output transistor, thereby the collector power dissipation of the output transistor being limited to a predetermined magnitude.
Prior art FIG. 1 shows an amplifier having such a protection circuit. The amplifier is formed in a complementary push-pull configuration comprising NPN transistors 1a, 2a and 3a in Darlington connection and PNP transistors 1b, 2b and 3b in Darlington connection. In this amplifier, the transistors 3a and 3b are adapted to operate as output transistors. Reference numerals 4a and 4b designate protection circuits for the respective NPN transistors and PNP transistors.
In the protection circuit 4a for the NPN transistors 1a, 2a and 3a, a base of the transistor 1a is connected to an output terminal 5 via a diode 10a and a NPN transistor 11a connected in series and in a forward direction, an emitter of the output transistor 3a is connected to the output terminal 5 via an emitter resistor 6a (resistance value: Re) and to a base of the transistor 11a via a resistor 7a (resistance value: R1), and the base of the transistor 11a is connected to the ground via a resistor 8a (resistance value: R2) and a diode 9a in a forward direction and in this order.
The protection circuit 4b for the PNP transistors 1b, 2b and 3b in a negative side is formed in a same manner as the protection circuit 4a, and corresponding parts are designated using same reference numerals as those of the circuit 4a with suffix of b.
In the protection circuit 4a, upon an assumption that an output voltage at the output terminal 5 is Vo, and that an output current of the amplifier or a collector current of the transistor 3a is Io, a voltage potential at the base of the transistor 11a are presented as follows: ##EQU1## wherein a forward voltage drop of the diode 9a is ignored.
Then, a protection operation of the protection circuit 4a is explained as follows. When the voltage difference between the voltage V1 and the output voltage Vo becomes greater than a base-emitter voltage V.sub.BE of the transistor 11a, namely when the following inequality (2) is established, the transistor 11a becomes conductive. ##EQU2## Accordingly, the base current of the transistor 1a which is applied with an input signal is bypassed to the output terminal 5 via the diode 10a and the transistor 11a, thereby the input signal being limited to a predetermined magnitude, with the result that the collector current of the output transistor 3a is reduced so as to limit the collector power dissipation of the output transistor 3a to a predetermined magnitude.
The protection operation of the protection circuit 4b in the negative side is performed in a same manner as that of the protection circuit 4a for the output transistor 3b.
From the inequality (2) above, a collector power dissipation limitation characteristics of the protection circuits 4a and 4b are expressed as following equation. ##EQU3## Accordingly, the characteristics are also expressed as solid lines A and B shown in FIG. 2. In detail, as shown in FIG. 2 by the solid line A, a limitation level of the output current Io flowing through the output transistor 3a becomes (R1/Re.multidot.R2).times.V.sub.BE when the output voltage Vo becomes null, for example, when a load connected to the output terminal 5 is short-circuited, and when the output voltage Vo increases in a positive direction from the null condition, the limitation level of the output current Io increases linearly by a gradient of R1/(Re.multidot.R2), and on the other hand when the output voltage Vo increases in a negative direction from the null condition, the limitation level of the output current Io is maintained in a constant level of (R1/Re.multidot.R2).times.V.sub.BE due to a non-conductiveness of the diode 9a.
A limitation level of the output current Io flowing through the output transistor 3b is presented by the solid line B shown in FIG. 2 based on a reason similar as that with respect to the output transistor 3a. It should be noted here that reference symbols A' and B' designate areas where protections of the respective output transistors 3a and 3b are performed.
To the output terminal 5 is ordinarily connected a loudspeaker having a reactance as a load of the amplifier. Therefore, relation between the output voltage Vo and the output current Io is expressed by a short dashes line C in FIG. 2 under the influence of a reactance component of the loudspeaker. It should be noted here that impedance phase deviation of a loudspeaker appears by 35.degree. to 40.degree. to the extent with respect to a resonance frequency f.sub.0 of the loudspeaker. Accordingly, when the protection circuit 4a and 4b are utilized in the amplifier, waveform distortion is developed in hatched areas shown in FIG. 2.
To solve the above-mentioned problem of waveform distortion, it is imagined that the resistance values R1 and R2 are adjusted at appropriate values so that the limitation levels of the output current Io under the condition of Vo=0 go out of the hatched area in FIG. 2, namely that the collector power dissipation characteristics of the protection circuits 4a and 4b are arranged as shown by solid lines A and B in FIG. 3.
However, another problem is developed under the above-mentioned solution as follows. When the load is continuously short-circuited, the current Io of enlarged limitation level under the condition of Vo=0 flows through the output transistors 3a and 3b continuously for a long period of time, with the result that the transistors 3a and 3b are destroyed due to over-currents flowing through the transistors 3a and 3b.
In this solution, it is able to solve the destruction problem by means of utilizing the output transistors with large current capacity and large radiators with large rediation capacity. However, this method has a disadvantage at the point of manufacture cost.